<PAGE>   1
                                                                     EXHIBIT 8.1

                   [WHYTE HIRSCHBOECK DUDEK S.C. LETTERHEAD]

                                 April 27, 1999

Board of Directors
Anchor BanCorp Wisconsin Inc.
25 West Main Street
Madison, WI  53707

           Re:    Merger of FCB Financial Corp. into Anchor BanCorp 
                  Wisconsin Inc.

Gentlemen and Ms. Cremer Berkenstadt:

         You have asked for our opinion with regard to the federal income tax
treatment of a proposed merger (the "Merger") of FCB Financial Corp. ("FCBF"), a
Wisconsin corporation, into Anchor BanCorp Wisconsin Inc. ("Anchor"), a
Wisconsin corporation, pursuant to an Agreement and Plan of Merger (the "Plan")
dated January 5, 1999 between Anchor and FCBF. This opinion is provided pursuant
to Section 7.2(e) of the Plan. As more fully described in the "S-4" (as defined
below) and the Plan, the Merger contemplates that FCBF will be merged with and
into Anchor, with the outstanding shares of FCBF being converted into shares of
common stock of Anchor, plus rights associated with such common stock under that
certain Rights Agreement between Anchor and Firstar Bank Milwaukee, N.A. (as
successor to Firstar Trust Company) dated as of July 22, 1997 (the "Anchor
Rights"). No fractional shares of Anchor will be issued in the Merger; rather,
fractional shares will be redeemed for cash as provided in the Plan.

         In rendering the opinions herein, we have reviewed the Plan, the
exhibits thereto, the Prospectus/Proxy Statement included as a part of the
Registration Statement on Form S-4 filed by Anchor and FCBF with the Securities
and Exchange Commission in connection with the transactions contemplated by the
Plan (the "S-4"), and the Application H-(e)3 filed by Anchor with the Office of
Thrift Supervision (the "Holding Company Application"). We have assumed:

         -            the accuracy of all of the statements made by the parties
                      in the Plan and the exhibits thereto and in the S-4 and
                      the Holding Company Application (other than the sections
                      thereof describing the federal income tax consequences of
                      the transactions);

         -            that the Merger will be carried out in accordance with the
                      terms of the Plan, as a merger of FCBF into Anchor
                      pursuant to the applicable provisions of the Wisconsin
                      Business Corporation Law ("WBCL"); and

<PAGE>   2


WHYTE                                                         Board of Directors
HIRSCHBOECK                                        Anchor BanCorp Wisconsin Inc.
DUDEK S.C.                                                        April 27, 1999
                                                                          Page 2

         -            the accuracy of the representations made to us by Anchor
                      and FCBF in their respective certificates to us, dated
                      this date (the "Representation Certificates"), and the
                      continued accuracy of those statements through the
                      Effective Time of the Merger.

         Based upon and subject to the foregoing, and subject to such other
qualifications as are hereinafter expressed, we are of the opinion that, for
federal income tax purposes:

         A.           The Merger will constitute a reorganization described in
                      Section 368(a)(1)(A) of the Code, and Anchor and FCBF will
                      be "parties to a reorganization" within the meaning of
                      Section 368(b) of the Code.

         B.           No gain or loss will be recognized by those shareholders
                      of FCBF whose stock in FCBF is converted into stock of
                      Anchor (plus associated Anchor Rights) as a result of the
                      Merger, except that those shareholders of FCBF who receive
                      cash in lieu of fractional shares will be required to
                      recognize gain attributable to such fractional shares,
                      determined as though such fractional shares had been
                      redeemed. Provided that those FCBF shareholders held their
                      FCBF stock as a capital asset on the effective date of the
                      Merger, the gain they recognize will be capital gain,
                      measured by the excess, if any, of the cash they receive
                      over that portion of their basis in their shares that is
                      properly allocated to the fractional share deemed to have
                      been redeemed.

         C.           Each shareholder of FCBF whose shares of stock in FCBF are
                      converted into stock of Anchor pursuant to the Merger will
                      have as the basis of his or her Anchor stock received in
                      the Merger the same basis as he or she had in his or her
                      stock in FCBF immediately prior to the Merger, adjusted
                      for any fractional shares deemed to have been redeemed as
                      referred to in the preceding paragraph.

         D.           The holding period for the stock of Anchor so acquired by
                      a former FCBF shareholder will include the period during
                      which the shareholder held his or her corresponding shares
                      of stock in FCBF, provided such corresponding shares of
                      stock in FCBF were held by the shareholder as a capital
                      asset on the effective date of the Merger.

         E.           No gain or loss will be recognized by FCBF or Anchor as a
                      result of the Merger.

         The opinions herein are limited to the federal income tax consequences
of the Merger as set forth above under the Code and the applicable
administrative, regulatory and judicial interpretations thereof as they exist as
of the date hereof. The provisions of the Code and any of such administrative,
regulatory and judicial interpretations could be changed at any time, and 


<PAGE>   3
WHYTE                                                         Board of Directors
HIRSCHBOECK                                        Anchor BanCorp Wisconsin Inc.
DUDEK S.C.                                                        April 27, 1999
                                                                          Page 3


such changes could be retroactively effective with respect to transactions
occurring prior to such changes, and accordingly could change our opinions
herein. The Internal Revenue Service is not bound by our opinions and may assert
positions contrary to those opinions. While our opinions herein reflect our
interpretations of existing sources of applicable law and what we expect a court
would conclude if asked to apply that law, no assurance can be given that our
interpretations would prevail if they were challenged in an administrative or
judicial proceeding

         No opinion is expressed as to any federal tax consequences of the
Merger other than those expressly set forth above, nor on the tax treatment or
the effect of the Merger (on FCBF, Anchor, or their respective shareholders)
under the laws of any state or other jurisdiction.

         As stated above, our opinions herein are based on the accuracy of the
assumptions stated herein and the factual representations contained in the
Representation Certificates. Any inaccuracy in any of such assumptions or
factual representations could render the opinions stated herein inapplicable and
result in the Merger's having tax consequences that are different than those
described herein.

         The opinions herein are rendered to you in connection with the Merger,
for the benefit of Anchor and FCBF and their respective shareholders, and are
not to be relied upon by any other persons or for any other purposes.

         We hereby consent to the filing of this opinion with the Securities and
Exchange Commission as an exhibit to the S-4 and to the reference to our firm
under the heading "THE MERGER - Federal Income Tax Consequences" in the Proxy
Statement/Prospectus filed as a part of the S-4. In giving such consent we do
not admit that we are "experts" within the meaning of Section 11 of the
Securities Act of 1933, as amended, or within the category of persons whose
consent is required by Section 7 of said Act.

                                        Very truly yours,

                                        WHYTE HIRSCHBOECK DUDEK S.C.

     
                                              
                                        By:  /s/ John F. Emanuel             
                                           ----------------------------------
                                             John F. Emanuel